The structure of the skull is a highly detailed and complex design. In all, there are 22 bones comprising the entire skull, excluding the 3 pairs of ossicles located in the inner ear. The bones of the skull are highly irregular. Most of the bones of the skull are held together by firm, immovable fibrous joints called sutures or synarthroses. These joints allow the developing skull to grow both pre- and postnatally.
The sutures of the skull are morphologically distinct, being divided into three main groups based on the margins of the articulating bones. At a simple suture, the margins of the articulating bones are smooth and meet end to end such as the median palatine suture. A bevelled suture is where the border of one bone overlaps that of the other like the parietotemporal suture. Conversely, the borders of the bones may have a number of projections that interdigitate with each other, giving a serrated appearance, such as in the sagittal suture. The complexity of the serrations of the sutures increases from internal to external.
|Posterior view||Lambdoid suture|
|Superior view||Coronal suture, sagittal suture|
|Lateral view||Pterion, sphenosquamous, squamous, parietomastoid, sphenoparietal, sphenofrontal, frontozygomatic, sphenozygomatic, occipitomastoid, temporozygomatic|
|Anterior view||Intermaxillary, zygomaticomaxillary, frontonasal, internasal, nasomaxillary, frontomaxillary, frontolacrimal, lacrimomaxillary|
|Base of skull||Median and transverse palatine sutures, spheno-occipital synchondrosis, petro-occipital suture, petrosphenoidal suture|
|Posterior view||Occipitomastoid suture|
|Synovial joints||Atlanto-occipital and temporomandibular joints|
The joints of the base of the skull are largely cartilaginous joints, or synchondroses. One such joint is the spheno-occipital synchondrosis, which is found between the body of the sphenoid anteriorly and the basilar part of the occipital bone posteriorly.
There are only two areas on the skull where synovial joints are present. The first is a pair of joints; the temporomandibular joints, where the mandible articulates with the skull on either side. The second synovial joint is the atlanto-occipital joint, where the base of the skull articulates with the vertebral column. Both these joints will be discussed further in this article. For the purpose of this article, the joints of the skull will be discussed by order of anatomical view, with the addition of the synovial joints being discussed separately.
The lambdoid suture can be found between the posterior border of the parietal bones and the anterolateral borders of the occipital bone . It is named so due to the lambdoid (λ) shape it forms with the coronal suture. It extends from the posterior extremity of the sagittal suture in a posteroinferior direction to meet the occipitomastoid suture from behind and the parietomastoid suture from above the mastoid process.
Sutural bones, also called wormian bones, are commonly found along the extent of the lambdoid suture. They form due to separate ossification centres to the lambdoid suture. A large interparietal bone, sometimes called the Inca bone, is often found between the posterior borders of the parietal bones.
The coronal suture lies between the posterior border of the frontal bone and the anterior margins of the left and right parietal bones. It projects inferiorly to meet the junction of the greater wing on the sphenoid bone and the squamous part of the temporal bone.
The sagittal suture runs in the midline, separating the left and right parietal bones from each other. It extends from the coronal suture anteriorly to the lambdoid suture posteriorly. The junction between the sagittal suture and the coronal suture is an area called the bregma, and was once a membranous portion of the developing skull called the anterior fontanelle.
The pterion is an H shaped region on the lateral aspect of the skull where a number of bones unite with each other. Participants of the pterion are the frontal bone, the parietal bone, the greater wing of the sphenoid bone, and the anterior of the squamous part of the temporal bone. Consequently, a number of sutures converge at the area. The sphenoparietal suture separates the superior surface of the greater wing of the sphenoid bone from the parietal bone above. The coronal suture joins the sphenoparietal suture superiorly, the sphenofrontal suture anteriorly, the sphenosquamous suture inferiorly, and the squamous suture posteriorly. Due to the extensiveness of the articulations at the pterion, this portion of the skull is weaker than other parts, and therefore more vulnerable to fracture.
The sphenosquamous suture is a vertical suture between the anterior margin of the squamous part of the temporal bone and the posterior margin of the greater wing of the sphenoid bone.
The squamous suture is a large horizontal suture that curves inferiorly as it passes towards the posterior aspect of the skull. It extends from the pterion anteriorly in a posteroinferior fashion to meet the parietomastoid suture. It separates the superior and posterior borders of the squamous part of the temporal bone from the posteroinferior angle of the parietal bone.
The parietomastoid suture lies between the inferior border of the angle of the parietal bone posteriorly and the mastoid process of the temporal bone. This suture is another common site for sutural bones to form.
The sphenoparietal suture is the articulation of the superior margin of the greater wing of the sphenoid bone and the parietal bone at the angle formed by the convergence of the coronal and squamous sutures at the pterion. The sphenoparietal suture separates the coronal and squamous sutures from each other at this point.
The sphenofrontal suture is an obliquely oriented suture extending anteroinferiorly between the lower margin of the frontal bone superiorly and the upper margin the greater wing of the sphenoid bone inferiorly.
The frontozygomatic suture is a horizontal suture lateral to the orbit. It lies between the frontal process of the zygomatic bone inferiorly and the zygomatic process of the frontal bone superiorly. There is a palpable depression at this suture.
The sphenozygomatic suture is the joint between the posteromedial aspect of the zygomatic bone anteriorly and the anterior margin of the greater wing of the sphenoid bone posteriorly.
The occipitomastoid suture is between the squamous part of the occipital bone and the mastoid process of the temporal bone. It extends anteroinferiorly towards the tip of the mastoid process.
The temporozygomatic suture is a vertical suture located just superior to the coronoid process of the mandible and inferior to the greater wing of the sphenoid bone when the skull is viewed from the lateral aspect. It is the site of articulation between the zygomatic process of the temporal bone posteriorly and the temporal process of the zygomatic bone anteriorly. The temporozygomatic suture completes the zygomatic arch.
The intermaxillary suture, which is located in the midline, is a vertical suture extending from the nasal aperture to the oral cavity below. It also extends posteriorly, and can be viewed as a horizontal suture on the anterior part of the hard palate, joining the palatine processes of the maxillae together in the roof of the mouth. The intermaxillary suture is continuous with the median palatine suture.
The zygomaticomaxillary suture is an oblique suture passing inferolaterally from the inferior aspect of the orbit, beginning just above the infraorbital foramen. It joins the thick, short zygomatic process of the maxilla and the zygomatic bone.
The frontonasal suture is located between the orbits on the superior aspect of the nose. It passes obliquely downwards from the midline, separating the upper border of the nasal bone from the inferior border of the nasal part of the frontal bone.
The internasal suture is a vertical, midline suture extending in a superior to inferior direction along the bridge of the nose. It extends from the frontonasal sutures superiorly to the superior aspect of the anterior nasal aperture. Where the internasal suture meets the frontonasal suture, there is a clear depression called the nasion which indicates the root of the nose. This depression is clearly visible on the surface.
The nasomaxillary suture is a vertical suture located between the lateral border of the nasal bone and the medial border of the frontal process of the maxilla. It extends inferiorly from the junction of the frontonasal and frontomaxillary sutures superiorly to the lateral margin of the anterior nasal aperture.
The frontomaxillary suture is an oblique suture between the frontal process of the maxilla, which extends upwards laterally to the lacrimal bone, and the nasal part of the frontal bone. It is continuous superiorly with the frontonasal suture.
The frontolacrimal suture is a small horizontal suture located in the medial wall of the orbit. It separates the frontal bone above from the superior border of the lacrimal bone below. It is a posterolateral continuation of the frontomaxillary suture.
The lacrimomaxillary suture is located in the medial wall of the orbit. It extends inferiorly and anteriorly towards the inferior border of the orbit, separating the anterior border of the lacrimal bone from the maxilla.
Base of Skull
For descriptive purposes, the base of the skull is divided into anterior, middle and posterior regions.
The anterior region of the base of the skull consists mainly of the hard palate, forming the roof of the oral cavity.
A midline suture, the median palatine suture, runs anteroposteriorly on the palate between the adjacent maxillary and palatine bones separating the palate into right and left sides. It is continuous anteriorly with the intermaxillary suture.
The transverse palatine suture is a transversely passing suture extending across the palate, separating the maxillae anteriorly from the palatine bones posteriorly.
The middle part of the cranial base extends from the posterior opening of the nares anteriorly as far as the anterior margin of the foramen magnum posteriorly.
The spheno-occipital synchondrosis is a cartilaginous joint that lies between the body of the sphenoid bone anteriorly and the basilar part of the occipital bone posteriorly.
The petro-occipital suture extends from medial to lateral as far as the jugular foramen. It separates the jugular fossa on the petrous portion of the temporal bone from the jugular process on the basilar part of the occipital bone.
The petrosphenoidal suture is located between the petrous portion of the temporal bone and the infratemporal surface of the greater wing of the sphenoid bone. The groove for the pharyngotympanic tube is also located between these two areas. The apex of the petrous process does not extend entirely as far as the spheno-occipital suture, resulting in an opening called the foramen lacerum medially.
The occipitomastoid suture, as previously discussed, is located on the posterior part of the base of the skull.
There are two different synovial joints present in the skull. One of them is the atlanto-occipital joint. This is where the condyles on the inferior surface of the occipital bone articulate directly with the C1 vertebra (the atlas). Flexion and extension of the skull occurs here, allowing the nodding motion of the head as in when we are saying yes.
As with all synovial joints, a number of ligaments stabilize the atlanto-occipital joint. These are the atlanto-occipital membranes. The posterior atlanto-occipital membrane extends from the posterior margin of the foramen magnum of the occipital bone to the posterior arch of the atlas. The anterior atlanto-occipital membrane extends from the anterior margin of the foramen magnum to the upper border of the anterior arch of the atlas.
The second of the synovial joints is a pair of joints located on the lateral sides of the skull. These are the temporomandibular joints. They are the points at which the head of the mandible, also called the condyle, articulates with the mandibular fossa of the temporal bone on either side of the skull.
Between the condyles of the mandible and the mandibular fossa is the articular disk, a small piece of cartilage. This disk effectively divides the joint into two separate joints. The upper joint is between the mandibular fossa of the temporal bone and the articular disk where side to side movement of the jaw (excursion) occurs. The lower joint is between the articular disk and the condyle of the mandible; this is the joint at which there is the hinge-like motion in opening and closing the mouth.
Reinforcing the temporomandibular joint are 3 ligaments. The temporomandibular ligament is a thickening of the lateral aspect of the joint capsule, extending from the zygomatic process and articular ligament of the temporal bone to the neck of the mandible. The stylomandibular ligament attaches to the styloid process of the temporal bone to the angle of the mandible, while the sphenomandibular ligament extends from the spine of the sphenoid bone to the lingula on the inner surface of the mandible.
The bones of the neurocranium develop as two different portions: the membranous part forming the flat bones and the cartilaginous part forming the bones of the base of the skull. At birth, the flat bones are separated from each other by sutures. At sites where more than two bones meet, these sutures are wide and incomplete. These areas are called fontanelles, more commonly known as ‘soft spots’. The anterior fontanelle is the most prominent of these structures. It is located where the parietal bones meet the frontal bone.
These fontanelles are present to allow the bones of the cranial vault to overlap during birth, and allow the skull to expand with brain growth after birth. Some remain open for a considerable time after birth, despite the cranium being fully structurally developed to full capacity between 5 and 7 years old; some of the sutures are not entirely complete until adulthood. The anterior fontanelle can be used clinically to determine if ossification of the sutures is progressing sufficiently. Anterior fontanelle ossification typically occurs around 18 to 24 months after birth, usually being the last fontanelle to close.
The skull is an extremely strong structure designed to protect the brain. Therefore, the shape and the biomechanics of the skull do the utmost to prevent fractures. In general, skull fractures have more clinical implications in relation to the extent of the force and nature of injury to the skull. The cause of the fracture(s) can be used to determine appropriate treatment in anticipation of more complex secondary complications.
A depressed skull fracture is more problematic. This is where a damaged bone protrudes further inward than the normal shape of the skull. This can cause a bleed, or haematoma, as well as a possible primary brain injury.
A compound fracture is where there is both a fracture of the bone and breach of the skin. This can be problematic as it exposes the site of injury to a greater risk of infection, with complications including meningitis. While the more typical presentation is a scalp laceration, compound fractures of sinuses can also occur, and are often undetected, which can causing intracranial infections post trauma.
The pterion is highly susceptible to fracture. Passing deep to the pterion is a large branch of the maxillary artery supplying the dura : the middle meningeal artery. Fracture at the pterion can produce a complete laceration of this artery and its accompanying vein resulting in bleeding into the epidural space between the dura mater and the skull. This results in an epidural hemorrhage or haematoma.